Connection unit, optical fiber line unit, optical cable, and optical transmission system

ABSTRACT

It is an object of this invention to provide a connection unit capable of improving transmission characteristics. The connection unit according to this invention is provided between a plurality of first optical fibers extending from an optical amplifier and a plurality of second optical fibers in an optical cable, and comprises one or more optical components to connect one first optical fiber to one second optical fiber. Each of the optical components includes third and fourth optical fibers that can be connected to one first optical fiber and one second optical fiber with low connection losses, respectively, and a loss adjusting section for adjusting a transmission loss. The transmission loss deviation in a plurality of optical transmission lines is adjusted to 0.005 dB/km or less.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a connection unit providedbetween an optical amplifier and optical cable, an optical fiber lineunit accommodated in an optical cable, an optical cable, and an opticaltransmission system including an optical cable and optical amplifier.

[0003] 2. Related Background Art

[0004] In a submarine optical cable or the like, to transmit opticalsignals over a long distance, repeaters are provided at predetermineddistance intervals (about 90 km at maximum; about 50 km in general) toamplify a weakened optical signal.

[0005] An optical cable has a plurality of optical fiber lines in anoptical cable sheath portion, and the plurality of optical fiber linesare respectively connected to a plurality of optical fibers extendingfrom an optical amplifier (to be also referred to as an optical amphereinafter) mounted in each repeater. A plurality of optical signalspropagating through the plurality of optical fiber lines collectivelystrike the optical amp via the plurality of optical fibers extendingfrom the optical amp to be collectively amplified.

SUMMARY OF THE INVENTION

[0006] The present inventors have found the following problems uponexamining the above conventional technique. An optical amp has aso-called gain tilt property, i.e., the property of having differentgains depending on the wavelengths of incident optical signals. Thisgain tilt changes depending on the power of an incident optical signal.FIG. 11 shows how the gain tilt changes depending on the incident power.Referring to FIG. 11, reference symbol P1 denotes the direction in whichthe incident power decreases; and P2, the direction in which theincident power increases. As shown in FIG. 11, when the incident poweris low, the gain on the long-wavelength side is higher than that on theshort-wavelength side, and vice versa.

[0007] In this case, signals having various wavelengths are transmittedthrough each optical fiber line owing to WDM (Wavelength DivisionMultiplex) transmission. Consequently, signals vary in power dependingon the wavelengths due to the gain tilt, and the transmissioncharacteristics deteriorate.

[0008] In this case, it is relatively easy to correct a gain tilt withrespect to one incident light beam. However, optical signals propagatingthrough the respective optical fiber lines are collectively incident onthe optical amp. Since the respective incident optical signals havepropagated through a plurality of optical fiber lines with differenttransmission losses (e.g., losses in the optical fibers themselves orlosses due to side pressures and bending), the powers of the respectiveoptical signals differ from each other. If the deviations amongtransmission losses in the respective optical fiber lines are large, thedeviations among the powers of the incident light beams are large.Therefore, gain tilts with respect to a plurality of optical signalsvary, and it is difficult to suppress the gain tilts with respect to alloptical signals to a predetermined value or less, resulting in adeterioration in transmission characteristics.

[0009] In a submarine optical cable, in particular, since opticalamplification is performed by optical amps on the respective opticalfiber lines at predetermined distance intervals, if the deviations amongtransmission losses in the respective optical fiber lines are large, thegain tilt increases every time each optical signal passes through anoptical amp. This greatly affects the transmission characteristics.

[0010] It is therefore an object of the present invention to provide aconnection unit, optical fiber line unit, optical cable, and opticaltransmission system which can improve transmission characteristics.

[0011] A connection unit according to the present invention is providedbetween a plurality of first optical fibers extending from an opticalamplifier and a plurality of second optical fibers in an optical cable.The connection unit comprises at least one optical component to connectone first optical fiber selected from the plurality of first opticalfibers to one second optical fiber selected from the plurality of secondoptical fibers. Each of the optical components comprises a third opticalfiber that can be connected to the selected first optical fiber with alow connection loss, a fourth optical fiber that can be connected to theselected second optical fiber with a low connection loss, and a lossadjusting section for adjusting a transmission loss of an opticaltransmission line including the selected first and second opticalfibers. The connection unit adjusts a difference between a highesttransmission loss and a lowest transmission loss among transmissionlosses in a plurality of optical transmission lines each including oneof the plurality of first optical fibers and one of the plurality ofsecond optical fibers to not more than 0.005 dB/km.

[0012] Since the connection unit can adjust the deviation among thetransmission losses in a plurality of optical transmission lines to0.005 dB/km or less, the powers of a plurality of optical signalsincident on the optical amplifier can be almost equalized. This makes itpossible to suppress the influences of the gain tilts and improve thetransmission characteristics.

[0013] The connection unit according to the present invention may becharacterized in that the loss adjusting section of the opticalcomponent includes a connection portion between the third and fourthoptical fibers, and the transmission loss is adjusted by a connectionloss at the connection portion.

[0014] The connection unit according to the present invention may becharacterized in that the third and fourth optical fibers of the opticalcomponent are fusion-spliced to each other at the connection portion,and a central axis of a core portion of the third optical fiber isoffset from a central axis of a core portion of the fourth optical fiberat the connection portion.

[0015] The connection unit according to the present invention may becharacterized in that the third and fourth optical fibers of the opticalcomponent are fusion-spliced to each other, and a core portion of atleast one of the third and fourth optical fibers is enlarged at theconnection portion.

[0016] The connection unit according to the present invention may becharacterized in that the. loss adjusting section of the opticalcomponent includes a bent portion formed on at least one of the thirdand fourth optical fibers, and the transmission loss is adjusted by abending loss at the bent portion.

[0017] The connection unit according to the present invention may becharacterized in that the loss adjusting section of the opticalcomponent includes an optical attenuator, and the transmission loss isadjusted by the optical attenuator.

[0018] An optical fiber line unit according to the present inventioncomprises a plurality of optical fibers accommodated in an optical cablesheath portion and constituting an optical cable together with theoptical cable sheath portion, each of the optical fiber lines having alength of not less than 30 km. Each of the optical fiber lines is formedby connecting a plurality of optical fibers. A difference between ahighest transmission loss and a lowest transmission loss amongtransmission losses in the optical fiber lines is not more than 0.005dB/km.

[0019] According to this optical fiber line unit, since the differencebetween transmission losses in a plurality of optical fiber lines is0.005 dB/km or less, the powers of a plurality of optical signalsincident on the optical amplifier are almost equalized. This makes itpossible to suppress the influences of gain tilts and improve thetransmission characteristics.

[0020] An optical cable according to the present invention isconstituted by a plurality of optical transmission lines each includingan optical fiber line having a first mode field diameter. At least oneof the plurality of optical transmission lines includes an optical fiberconnected to one end of the optical fiber line and having a second modefield diameter smaller than the first mode field, and a loss adjustingsection for adjusting an overall transmission loss in the selectedoptical transmission line including the optical fiber line and theoptical fiber. A difference between a highest transmission loss and alowest transmission loss among transmission losses in the plurality ofoptical transmission lines is not more than 0.005 dB/km.

[0021] An optical cable according to the present invention isconstituted by a plurality of optical transmission lines each includingan optical fiber line having a portion having, a first mode fielddiameter and a portion having a second mode field diameter larger thanthe first mode field diameter. At least one optical transmission lineselected from the plurality of optical transmission lines includes anoptical fiber connected to one end of the optical fiber line and havinga third mode field diameter larger than the first mode field diameterand smaller than the second mode field diameter, and a loss adjustingsection for adjusting an overall transmission loss in the selectedoptical transmission line including the optical fiber line and theoptical fiber. A difference between a highest transmission loss and alowest transmission loss among transmission losses in the plurality ofoptical transmission lines is not more than 0.005 dB/km.

[0022] In this optical cable, since the deviation between thetransmission losses in a plurality of optical transmission lines is0.005 dB/km or less, the powers of a plurality of optical signalsincident on the optical amplifier are almost equalized to suppress theinfluences of gain tilts, and the transmission characteristics can beimproved.

[0023] An optical transmission system according to the present inventioncomprises an optical cable having a plurality of optical fiber lines, anoptical amplifier, and a plurality of optical fibers extending from theoptical amplifiers At least one of optical transmission lines includingone optical fiber line selected from the plurality of optical fiberlines and one optical fiber selected from the plurality of opticalfibers includes a loss adjusting section for adjusting a differencebetween a highest transmission loss and a lowest transmission loss amongtransmission losses in the plurality of optical transmission lines tonot more than 0.005 dB/km.

[0024] In this optical transmission system, since the deviation betweenthe transmission losses in a plurality of optical transmission lines canbe adjusted to 0.005 dB/km or less, the powers of a plurality of opticalsignals incident on the optical amplifier are almost equalized tosuppress the influences of gain tilts, and the transmissioncharacteristics can be improved.

[0025] The present invention will be more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only and are not to be consideredas limiting the present invention.

[0026] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1A is a side view of an optical transmission system;

[0028]FIG. 1B is a schematic view for explaining the connection statebetween an optical cable and an optical amplifier;

[0029]FIG. 2 is a side view showing an embodiment of a connection unitaccording to the present invention;

[0030]FIG. 3A is a side view showing an example of an optical component;

[0031]FIGS. 3B and 3C are side views showing an example of theconnection portion of the optical component;

[0032]FIGS. 4A and 4B are side views showing an example of the opticalcomponent;

[0033]FIG. 5 is a side view showing an example of the optical component;

[0034]FIG. 6A is a side view showing an embodiment of an optical fiberline unit according to the present invention;

[0035]FIG. 6B is a side view showing an optical cable having an opticalfiber line unit according to the present invention;

[0036]FIG. 7 is a side sectional view showing an embodiment of anoptical cable according to the present invention;

[0037]FIG. 8 is a side sectional view showing another embodiment of anoptical cable according to the present invention;

[0038]FIG. 9 is a side view showing the connection state between theoptical cable and the optical amplifier in FIG. 8;

[0039]FIGS. 10A, 10B, and 10C are side views showing an embodiment of anoptical transmission system according to the present invention in only apartial section; and

[0040]FIG. 11 is a graph showing the relationship between the wavelengthand the gain to explain a gain tilt in an optical amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] In the following, preferred embodiments of a connection unit,optical fiber line unit, optical cable, and optical transmission systemin accordance with the present invention will be explained in detailwith reference to the accompanying drawings. Here, constituentsidentical to each other will be referred to with numerals identical toeach other, without repeating their overlapping explanations.

[0042] The connection unit, optical fiber line unit, optical cable, andoptical transmission system according to this embodiment can be suitablyused for a submarine optical transmission system or the like. As shownin FIG. 1A, in an optical cable 101 of an optical transmission system 90such as a submarine optical transmission system, in order to transmit anoptical signal for a long distance, repeaters 100 are provided atpredetermined distance D (about 90 km at maximum; about 50 km ingeneral) intervals to amplify a weakened optical signal. As shown inFIG. 1B, each repeater 100 incorporates an optical amplifier (to be alsoreferred to as an optical amp hereinafter) 102. An optical signal isamplified by this optical amp 102.

[0043]FIG. 1B schematically shows the connection state between theoptical amp 102 and optical fibers 103 in the optical cable 101. Eachoptical fiber 103 in the optical cable 101 is connected to acorresponding one of a plurality of optical fibers 104 extending fromthe optical amp 102 via a connection unit 80 according to thisembodiment. optical signals propagating from the plurality of opticalfibers 103 in the optical cable 101 to the plurality of optical fibers104 via the connection unit 80 are multiplexed into one signal by anoptical coupler or the like and collectively amplified by an amplifyingsection 105 of the optical amp 102. The amplified optical signal isdemultiplexed into signals by an optical coupler or the-like. Thesesignals are then transmitted to a plurality of optical fibers 103 in anoptical cable 101 on the opposite side via an connection unit 80.

[0044] In this case, the optical fibers 104 extending from the opticalamp 102 are welded to a pressure barrier 102 a of the optical amp 102.For this reason, each optical fiber 104 is required to have heatresistance and resistance to bending. To realize heat resistance,polyimide resin or the like is used for the sheath of the optical fiber104. A metal is evaporated on the surface of the sheath of the opticalfiber 104 to improve its joining property in welding. The optical fiber104 is also required to have resistance to bending stress produced by,for example, welding to the pressure barrier 102 a. An optical fiberhaving a small mode field diameter (to be also referred to as an MFDhereinafter) is superior in resistance to bending to an optical fiberhaving a large MFD. As the optical fiber 104, therefore, an opticalfiber having a small MFD is preferably used.

[0045] In contrast to this, as each optical fiber 103 in the opticalcable 101, an optical fiber having a relatively large MFD is preferablyused to avoid a nonlinear phenomenon. In the nonlinear phenomenon, asthe intensity of light increases, distortion occurs. In order tosuppress this phenomenon, an increase in the power density of an opticalsignal is preferably suppressed. To decrease the power density byspreading the light within a cross section, an optical fiber having alarge MFD is preferably used. That is, the performance required for theoptical fiber 103 in the optical cable 101 differs from the performancerequired for the optical fiber 104 extending from the optical amp 102.Different types of optical fibers are therefore used for them.

[0046] In the optical transmission system 90 having the abovearrangement, in order to decrease the transmission loss by suppressingvariations in gain tilt between the respective optical transmissionlines, it is preferable that the powers of light striking the opticalamp be almost equalized. If the powers of light propagating through allthe optical transmission lines and striking the optical amp are almostequal, almost equal gain tilts are produced in all the opticaltransmission lines. This makes it easy to correct the gain tilts (whichneed not be corrected in some case). The connection unit, optical fiberline unit, optical cable, and optical transmission system according tothis embodiment are suitably used to almost equalize the powers of lightincident on the optical amp.

[0047] The connection unit provided between the optical fibers in theoptical cable and the optical fibers extending from the opticalamplifier (optical amp) will be described first.

[0048] As described above, since the performance required for theoptical fiber 103 in the optical cable 101 differs from that for theoptical fiber 104 extending from the optical amp 102, optical fibers ofdifferent types (MFDS) are used for them. It is difficult to connectoptical fibers having different MFDs to each other and adjust theconnection loss at the connection portion. In constructing a submarineoptical transmission system, in particular, since optical amps areconnected to optical cables on a cable-laying ship, it is difficult toadjust the connection losses at the connection portions.

[0049] As shown in FIG. 2, therefore, the connection units 80 eachhaving one or more optical components 107 are provided between theplurality of first optical fibers 104 extending from the optical amp 102and the plurality of second optical fibers 103 in the optical cables 101to almost equalize the powers of light propagating through therespective optical transmission lines 82, each including the firstoptical fibers 104 and second optical fibers 103, and striking theoptical amp 102. Each optical component 107 of the connection unit 80includes a third optical fiber 108 that can be connected to one firstoptical fiber 104 with a low connection loss and a fourth optical fiber109 that can be connected to one optical fiber 103 with a low connectionloss.

[0050] In this case, “can be connected to, . . . , with a low connectionloss” indicates that the connection loss does not increase even ifoptical fibers are connected by general fusion splicing. Morespecifically, this connection can be done like connection betweenoptical fibers of the same type or optical fibers having the same MFD.

[0051] A loss adjusting section 106 is formed between each third opticalfiber 108 and the corresponding fourth optical fiber 109. The lossadjusting section 106 adjusts the transmission loss in the opticaltransmission line 82 including the first optical fiber 104 and secondoptical fiber 103.

[0052] By using the connection unit 80 having one or more opticalcomponents 107 described above, the powers of optical signals incidenton the optical amp 102 can be almost equalized. More specifically, ifthe power of light emerging from a given optical transmission line 82 ishigher than that from another optical transmission line 82, the opticalcomponent 107 that is adjusted in advance to increase the transmissionloss is provided between the first optical fiber 104 and the secondoptical fiber 103, and the two optical transmission lines are connectedto each other. In contrast to this, for the optical transmission line 82from which light having a power lower than that from another opticaltransmission line 82 emerges, the optical component 107 that is adjustedin advance to decrease the transmission loss is provided between thefirst optical fiber 104 and the second optical fiber 103, and the twooptical transmission lines are connected to each other.

[0053] In this case, the optical components 107 need not be provided forall the optical transmission lines 82, and the optical component 107 maybe used for only the optical transmission line 82 from which lighthaving a very high power emerges. In this sense, it suffices if theconnection unit 80 includes “one or more optical components”.

[0054] This connection unit 80 controls the difference between thehighest transmission loss and the lowest transmission loss among thetransmission losses in the plurality of optical transmission lines 82,i.e., the transmission loss deviation, to 0.005 dB/km or less, and morepreferably, 0.002 dB/km or less. With this operation, the powers oflight incident on the optical amp 102 can be equalized, and variationsin gain tilt are suppressed, thus improving the transmissioncharacteristics. Note that, as described above, connection between thefirst optical fiber 104 and the third optical fiber 108 and between thesecond optical fiber 103 and the fourth optical fiber 109 is connectionwith a low transmission loss (e.g., connection between optical fibers ofthe same type) and hence hardly influence transmission loss adjustmentby the loss adjusting section 106 described above. Alternatively, someconsideration can be given to the influence of the connection.

[0055] A few specific examples of the loss adjusting section 106 in theoptical component 107 described above will be presented.

[0056] As shown in FIG. 3A, a case wherein the loss adjusting section106 has a connection portion 110 between the third optical fiber 108 andthe fourth optical fiber 109 will be described first. In this case, thetransmission loss in the optical transmission line 82 is adjusted by theconnection loss at the connection portion 110. When the transmissionloss in the optical transmission line 82 is to be adjusted by theconnection loss at the connection portion 110, the third optical fiber108 and the fourth optical fiber 109 are preferably axially shifted atthe connection portion 110.

[0057] More specifically, the third optical fiber 108 is preferablywelded to the fourth optical fiber 109 while a central axis 108 b of acore portion 108 a of the third optical fiber 108 is shifted from acentral axis 109 b of a core portion 109 a of the fourth optical fiber109. In this case, the connection loss can be increased by increasingthe axial shift amount, and vice versa.

[0058] Alternatively, when the transmission loss in the opticaltransmission line 82 is to be adjusted by the connection loss at theconnection portion 110, at least one of the core portions of the thirdand fourth optical fibers 108 and 109 is preferably enlarged. In thecase shown in FIG. 3c, the core portion 108 a of the third optical fiber108 is enlarged. When one of the core portions of the third and fourthoptical fibers 108 and 109 is to be enlarged, the connection loss can beincreased by increasing the diameter deviation of the core portion, andvice versa.

[0059] The transmission loss in the optical transmission line 82 may beadjusted by the bending loss at a bent portion 111 of the loss adjustingsection 106 instead of the connection loss at the connection portion 110of the loss adjusting section 106, as shown in FIGS. 4A and 4B. Thisbent portion 111 may be formed on the third optical fiber 108 as shownin FIG. 4A or on the fourth optical fiber 109 as shown in FIG. 4B. Whenthe transmission loss in the optical transmission line 82 is to beadjusted by the bent portion 111, the shape of the bent portion 111 ispreferably maintained by using an ultraviolet-curing resin or the like.In this case, the bent portion 111 may be fixed while being wound on amandrel, or the mandrel may be removed after the bent portion is fixed.

[0060] The transmission loss in the optical transmission line 82 may beadjusted by an optical attenuator 112 of the loss adjusting section 106instead of the connection loss and the bending loss.

[0061] An optical fiber line unit capable of improving transmissioncharacteristics will be described next.

[0062] An optical fiber line unit 70 is a set of optical fiber lines113. This set is prepared to be accommodated in an optical cable sheathportion 72. As shown in FIGS. 6A and 6B, each optical fiber line 113 isformed by connecting a plurality of optical fibers 114 to each other andhas a length L of 30 km or more. It is very difficult to manufacture aunitary optical fiber having a length of 30 km or more. For this reason,in manufacturing an optical cable 115, each optical fiber line 113 isformed by connecting a plurality of optical fibers 114 to each other.

[0063] In this optical fiber line unit 70, the difference between thehighest transmission loss and the lowest transmission loss among thetransmission losses in all the optical fiber lines 113, i.e., thetransmission loss deviation, is set to 0.005 dB/km or less, and morepreferably, 0.002 dB/km or less. In other words, the optical fibers 114are selected and connected (the connection losses in the process ofconnection are also adjusted as needed) so as to set the transmissionloss deviation to 0.005 dB/km or less, and more preferably, 0.002 dB/kmor less. Alternatively, the connection losses in connecting the opticalfibers 114 to each other are adjusted to set the transmission lossdeviation to 0.005 dB/km or less, and more preferably, 0.002 dB/km orless. If the optical cable 115 is manufactured by using this opticalfiber line unit 70, the transmission losses in all the optical fiberlines 113 can be almost equalized. This will contribute to equalizationof the powers of light incident on each optical amp.

[0064] The gain tilts produced in optical amps depend on thecharacteristics of erbium-doped fibers used for the optical amps, andhence generally vary depending on the optical amps. If, however, thetransmission loss deviation in the plurality of optical fiber lines 113described above is set to 0.005 dB/km or less, and preferably, 0.002dB/km or less, changes in optical signals due to the gain tilts can besuppressed to a level at which the transmission characteristics are notinfluenced. If the above transmission loss deviation exceeds 0.005dB/km, the gain tilts may affect the transmission characteristics.

[0065] An optical cable that can improve transmission characteristicswill be described next.

[0066]FIG. 7 shows the arrangement of the first optical cable. Thisoptical cable 116 is made up of a plurality of optical transmissionlines 141 each including an optical fiber line 117 having apredetermined MFD (D1) (FIG. 7 shows only one optical transmission line141). The plurality of optical transmission lines 141 are accommodatedin an optical cable sheath portion 116 a. At least one of the opticaltransmission lines 141 is connected to an optical fiber 119 having anMFD (D2) smaller than the MFD (D1) (D2<D1) of the optical fiber line 117via a loss adjusting section 118 formed at one end of the optical fiberline 117. This optical fiber 119 can be connected to an optical fiber120 extending from an optical amp with a low connection loss. Note thatthe loss adjusting section 118 has the same arrangement as that of theloss adjusting section 106 described with reference to FIGS. 2, 3A, 3B,3C, 4A, 4B, and 5.

[0067] The optical fiber of the optical fiber line 117 included in theoptical transmission line 141 has a relatively larger MFD to suppress anonlinear phenomenon. On the other hand, the optical fiber 120 extendingfrom the optical amp is welded to a pressure barrier, and hence has arelatively small MFD to reduce transmission loss due to bending. Whenoptical fibers having different MFDs are connected to each other, theconnection loss becomes large and unstable. This makes it difficult toobtain a desired transmission loss. For this reason, the loss adjustingsection 118 is set in at least one optical transmission line 141 toadjust the transmission loss to set the difference between the highesttransmission loss and the lowest transmission loss among thetransmission losses in the plurality of optical transmission lines 141,i.e., the transmission loss deviation, to 0.005 dB/km or less, and morepreferably, 0.002 dB/km or less.

[0068] In this manner, the transmission characteristics can be improvedby almost equalizing the transmission losses in the optical transmissionlines 141 as described in association with the connection unit 80.

[0069]FIG. 8 shows the arrangement of the second optical cable. Thisoptical cable 121 is comprised of a plurality of optical transmissionlines 151 each including an optical fiber line 122 having two portionshaving different MFDs (FIG. 8 shows only one optical transmission line151). These optical transmission lines 151 are accommodated in anoptical cable sheath portion 121 a. In this case, an optical fiber 122 ahaving a large MFD is set on the incident side of the optical fiber line122, and an optical fiber 122 b having a smaller optical fiber is set onthe exit side. A nonlinear phenomenon is reduced by the optical fiber122 a having a relatively larger MFD (D3). The transmission speed isincreased by combining this optical fiber 122 a with the optical fiber122 b, which has a relatively small MFD (D4) but reduces the dispersiontilt (at which the wavelength dispersion of the optical fiber changesdepending on the wavelength).

[0070] At least one of the optical transmission lines 151 is connectedto an optical fiber 124 having an MFD (D5) larger than the MFD (D4) ofthe optical fiber 122 b and smaller than the MFD (D3) of the opticalfiber 122 a (D4<D5<D3) via a loss adjusting section 123 connected to oneend (the exit side connected to the optical amp) of the optical fiberline 122. This optical fiber 124 can be connected to an optical fiber125 extending from the optical amp with a low connection loss. Note thatthis loss adjusting section 123 also has the same arrangement as that ofthe loss adjusting section 106 shown in FIGS. 2, 3A, 3B, 3C, 4A, 4B, and5.

[0071] As described above, the MFD of the optical fiber 125 extendingfrom the optical amp cannot be increased in order to prevent a bendingloss by welding at the pressure barrier. In addition, in the opticalfiber 125 extending from the optical amp, when light strikes the opticalamp, a nonlinear phenomenon is small because the optical power issufficiently attenuated. When, however, light emerges from the opticalamp, the optical power is strong. For this reason, in order to reducethe nonlinear phenomenon, the MFD of the optical fiber 125 is notpreferably decreased either. Therefore, the nonlinear phenomenon can bereduced and the transmission characteristics can be improved by settingthe MFD (D5) of the optical fiber 125 to be larger than the MFD (D4) ofthe optical fiber 122 b and smaller than the MFD (D3) of the opticalfiber 122 a and setting the MFD of the optical fiber 124 to be equal tothe MFD (D5) of the optical fiber 125. Note that the optical fiber 124can be connected to the optical fiber 125 extending from the optical ampwith a low connection loss (for example, optical fibers of the same typeare connected), and hence hardly influence the adjustment oftransmission loss by the loss adjusting section 123 (or the influence onthe adjustment can be considered in advance).

[0072]FIG. 9 shows a state wherein the above optical cable 121 isconnected to an optical amp 126. FIG. 9 shows only two of a plurality ofoptical transmission lines 151 in the optical cable 121. In one of theoptical transmission lines 151 shown in FIG. 9 (the upper opticaltransmission line in FIG. 9), an optical signal is transmitted in thedirection indicated by arrows S1 and S2 in FIG. 9. In the other opticaltransmission line (the lower optical transmission line in FIG. 9), anoptical signal is transmitted in the direction indicated by arrows S3and S4 in FIG. 9.

[0073] In this case, the loss adjusting section 123 described above isset in each optical transmission line 151. The difference between thehighest transmission loss and the lowest transmission loss among thetransmission losses in the plurality of optical transmission lines 151,i.e., the transmission loss deviation, is set to 0.005 dB/km or less,and more preferably, 0.002 dB/km or less to equalize the powers ofoptical signals incident on the optical amp 126. In addition, in eachoptical transmission line 151 in the optical cable 121, the opticalfiber 122 a having the larger MFD (D3) is provided on the incident side,and the optical fiber 122 b having the small MFD (D4) is provided on theexit side to have excellent transmission characteristics, as describedabove. The optical fiber 125 has the MFD (D5) larger than the MFD (D4)of the optical fiber 122 b and smaller than the MFD (D3) of the opticalfiber 122 a. The MFD of the optical fiber 124 is equal to the MFD (D5)of the optical fiber 125. This makes it possible to reduce the nonlinearphenomenon caused by a high-power optical signal emerging from theoptical amp 126.

[0074] An optical transmission system capable of improving transmissioncharacteristics will be described next.

[0075] The optical transmission system described below is comprised ofan optical cable and optical amp. As described above, in this opticaltransmission system, the transmission characteristics may deterioratedue to a gain tilt. As also described above, in order to suppress theadverse effect of this gain tilt, the transmission characteristics canbe improved by using the connection unit 80 including the opticalcomponent 107 having the loss adjusting section 106, which is shown inFIGS. 2, 3A, 3B, 3C, 4A, 4B, and 5. The optical transmission systemhaving this connection unit 80 between an optical cable and an opticalamp can improve its transmission characteristics.

[0076] This optical transmission system has the connection unit 80provided between the first optical fiber 104 and the second opticalfiber 103 as shown in FIG. 1B and has an arrangement like the one shownin FIG. 1A. A similar effect can be obtained by connecting a lossadjusting section to an optical fiber in an optical cable or an opticalfiber extending from an optical amp or providing a loss adjustingsection at the connection portion between an optical fiber on theoptical cable side and an optical fiber extending from an optical amp.FIGS. 10A to 10C show such cases.

[0077] In an optical transmission system (shown in only a partialsection) 160 shown in FIG. 10A, an optical fiber 130 that can beconnected to an optical fiber 129 in an optical cable 128 with a lowconnection loss is attached to the distal end of at least one of aplurality of optical fibers 127 extending from the optical amp 126. Aloss adjusting section 131 is formed on an optical transmission lineextending from this optical fiber 127 to the optical fiber 130. The lossadjusting section 131 has the same arrangement as that of the lossadjusting section 106 shown in FIGS. 2, 3A, 3B, 3C, 4A, 4B, and 5. Thedifference between the highest transmission loss and the lowesttransmission loss among the transmission losses in the plurality ofoptical transmission lines 162 including the optical fibers 127 and 129,i.e., the transmission loss deviation, is set to 0.05 dB/km or less, andmore preferably, 0.002 dB/km or less. With this setting, the powers oflight propagating through all the optical transmission lines 162 andstriking the optical amp 126 are almost equalized, and variations ingain tilt among the optical transmission lines 162 are suppressed, thusimproving the transmission characteristics.

[0078] In the optical transmission system 160 shown in FIG. 10B, anoptical fiber 132 that can be connected to the optical fiber 127extending from the optical amp 126 with a low connection loss isattached to the distal end of at least one of the plurality of opticalfibers 129 in the optical cable 128. A loss adjusting section 133 isformed on an optical transmission line extending from the optical fibers129 to the optical fiber 132. The loss adjusting section 133 has thesame arrangement as that of the loss adjusting section 106 shown inFIGS. 2, 3A, 3B, 3C, 4A, 4B, and 5. The difference between the highesttransmission loss and the lowest transmission loss among thetransmission losses in the plurality of optical transmission lines 162including the optical fibers 127 and 129, i.e., the transmission loss,is set to 0.005 dB/km or less, and more specifically, 0.002 dB/km orless. With this setting, the powers of light propagating through all theoptical transmission lines 162 and entering the optical amp 126 arealmost equalized, and variations in gain tilt among the opticaltransmission lines 162 are suppressed, thus improving the transmissioncharacteristics.

[0079] In the optical transmission system 160 shown in FIG. 10C (shownin only a partial section in FIG. 10C), the plurality of optical fibers127 extending from the optical amp 126 are respectively connected to theplurality of optical fibers 129 in the optical cable 128 via a lossadjusting section 134. This loss adjusting section 134 has the samearrangement as that of the loss adjusting section 106 shown in FIGS. 2,3A, 3B, 3C, 4A, 4B, and 5. The difference between the highesttransmission loss and the lowest transmission loss among thetransmission losses in the plurality of optical transmission lines 162including the optical fibers 127 and 129, i.e., the transmission lossdeviation, is set to 0.005 dB/km or less, and more preferably, 0.002dB/km or less. With this setting as well, the powers of lightpropagating through all the optical transmission lines 162 and enteringthe optical amp 126 are almost equalized, and variations in gain tiltamong the optical transmission lines 162 are suppressed, thus improvingthe transmission characteristics.

[0080] According to the embodiment described above, the influences ofgain tilts in connection to an optical amp on transmissioncharacteristics are suppressed. However, the present invention can beapplied to a case wherein the influences of gain tilts on transmissioncharacteristics pose a problem regardless of whether optical fibers areconnected to an optical amp.

[0081] According to the connection unit, optical fiber line unit,optical cable, and optical transmission system according to the presentinvention, the powers of optical signals incident on an opticalamplifier can be almost equalized. Since the powers of light incident onthe optical fiber can be almost equalized, the adverse effect of gaintilts can be suppressed, thus improving the transmissioncharacteristics.

[0082] From the invention thus described, it will be obvious that theinvention may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the invention, andall such modifications as would be obvious to one skilled in the art areintended for inclusion within the scope of the following claims.

What is claimed is:
 1. A connection unit provided between a plurality offirst optical fibers extending from an optical amplifier and a pluralityof second optical fibers in an optical cable, comprising not less thanone optical component to connect one first optical fiber selected fromthe plurality of first optical fibers to one second optical fiberselected from the plurality of second optical fibers, wherein each ofsaid optical components comprises a third optical fiber that can beconnected to the selected first optical fiber with a low connectionloss, a fourth optical fiber that can be connected to the selectedsecond optical fiber with a low connection loss, and a loss adjustingsection for adjusting a transmission loss of an optical transmissionline including the selected first and second optical fibers, saidconnection unit adjusts a difference between a highest transmission lossand a lowest transmission loss among transmission losses in a pluralityof optical transmission lines each including one of the plurality offirst optical fibers and one of the plurality of second optical fibersto not more than 0.005 dB/km.
 2. A unit according to claim 1; whereinsaid loss adjusting section of said optical component includes aconnection portion between the third and fourth optical fibers, and thetransmission loss is adjusted by a connection loss at the connectionportion.
 3. A unit according to claim 2, wherein the third and fourthoptical fibers of said optical component are fusion-spliced to eachother at the connection portion, and a central axis of a core portion ofthe third optical fiber is offset from a central axis of a core portionof the fourth optical fiber at the connection portion.
 4. A unitaccording to claim 2, wherein the third and fourth optical fibers ofsaid optical component are fusion-spliced to each other, and a coreportion of at least one of the third and fourth optical fibers isenlarged at the connection portion.
 5. A unit according to claim 1,wherein said loss adjusting section of said optical component includes abent portion formed on at least one of the third and fourth opticalfibers, and the transmission loss is adjusted by a bending loss at thebent portion.
 6. An unit according to claim 1, wherein said lossadjusting section of said optical component includes an opticalattenuator, and the transmission loss is adjusted by the opticalattenuator.
 7. An optical fiber line unit comprising a plurality ofoptical fibers accommodated in an optical cable sheath portion andconstituting an optical cable together with said optical cable sheathportion, each of said optical fiber lines having a length of not lessthan 30 km, wherein each of said optical fiber lines is formed byconnecting a plurality of optical fibers, and a difference between ahighest transmission loss and a lowest transmission loss amongtransmission losses in said optical fiber lines is not more than 0.005dB/km.
 8. An optical cable constituted by a plurality of opticaltransmission lines each including an optical fiber line having a firstmode field diameter, wherein at least one of said plurality of opticaltransmission lines includes an optical fiber connected to one end ofsaid optical fiber line and having a second mode field diameter smallerthan the first mode field, and a loss adjusting section for adjusting anoverall transmission loss in said selected optical transmission lineincluding said optical fiber line and said optical fiber, and adifference between a highest transmission loss and a lowest transmissionloss among transmission losses in said plurality of optical transmissionlines is not more than 0.005 dB/km.
 9. An optical cable constituted by aplurality of optical transmission lines each including an optical fiberline having a portion having a first mode field diameter and a portionhaving a second mode field diameter larger than the first mode fielddiameter, wherein at least one optical transmission line selected fromsaid plurality of optical transmission lines includes an optical fiberconnected to one end of said optical fiber line and having a third modefield diameter larger than the first mode field diameter and smallerthan the second mode field diameter, and a loss adjusting section foradjusting an overall transmission loss in said selected opticaltransmission line including said optical fiber line and said opticalfiber, and a difference between a highest transmission loss and a lowesttransmission loss among transmission losses in said plurality of opticaltransmission lines is not more than 0.005 dB/km.
 10. An opticaltransmission system comprising an optical cable having a plurality ofoptical fiber lines, an optical amplifier, and a plurality of opticalfibers extending from said optical amplifier, wherein at least one ofoptical transmission lines including one optical fiber line selectedfrom said plurality of optical fiber lines and one optical fiberselected from said plurality of optical fibers includes a loss adjustingsection for adjusting a difference between a highest transmission lossand a lowest transmission loss among transmission losses in saidplurality of optical transmission lines to not more than 0.005 dB/km.